The present invention relates generally to the field of x-ray inspection. More particularly the present invention relates to x-ray inspection of meat.
Inspection of various production products has become increasingly important in recent years. Traditionally, product inspection has been limited to physical inspection of the product by a worker on the production line. Obviously, this form of inspection is less than optimal. As such, two more useful devices were developed and became the standard inspection apparatus: a check weigher and a metal detector. Each of these devices has its own inherent limitations, and even the system in combination lacked the ability to provide much information. Therefore, a need exists for an inspection system that can provide more detailed and variable data. The types and breadth of inspection data needed vary from product to product.
One category of product for which inspection is especially important is food. Many properties of food need to be monitored and controlled such as but not limited to weight, temperature, amount of contaminants, nutrient levels, fat levels, and carbohydrate levels. In particular, the level of fat and carbohydrates included in diets is of concern in the current consumer market place. Awareness of fat intake has led consumers to value highly those food sources which are relatively low in fat or have virtually no fat content. This is especially true with respect to meat products or foods which contain animal-originating meat components. Meat products must be carefully inspected to ensure that the meat has the desired properties. Such properties include but are not limited to weight of the meat, meat tenderness, the effective atomic number of the meat, and the amount of contamination.
One characteristic which has become increasingly important to monitor is the meat yield. Meat yield is a measure of the percentage of a sample of meat that is fat and the percentage of the sample of meat that is chemical lean. Lean meat and meat fat have different chemical compositions. Lean meat has a high concentration of moisture and protein and includes nitrogen and oxygen atoms which are of a greater atomic number than the carbon and hydrogen atoms which predominate in meat fat.
Although techniques for chemically analyzing food products, such as for determining the amount of fat, are well known, such laboratory techniques are time consuming and costly. Moreover, these techniques typically require that the product be physically or chemically broken down, consequently, only selected samples of the product can be analyzed, rather than each product. This diminishes the accuracy of the analysis since the quantities of substances and contaminants can vary from one product to another.
Based on these and other chemical differences between lean meat and meat fat, devices for non-destructively determining the quantitative relationship between meat components by gamma radiation (x-rays) are known in the art. Such devices are based on the principle that x-rays are affected by the various components of the material in distinct, measurable ways. In general, a typical basic x-ray device is a linear array comprising a high voltage power supply to power a x-ray tube wherein a beam of x-rays is directed at the product. The x-ray beam passes through the product to ultimately impinge upon a sensor or sensors, such as a row of detector diodes. Such x-rays devices typically then display an image of the material based on the x-rays. This image can provide valuable information which a normal optical image cannot. The formation of images due to light or X-ray differs. The major difference is that optical images are created by light reflection on the object surface and X-ray images are formed due to X-rays absorption by passing through a material. Thus, an optical image gives information about the object's surface and an X-ray image supplies information about the inner structure of the object.
An X-ray image is a silhouette, where the degree of transparency is dependent on the density, thickness and the atomic number of the material. Using the current technology this information can be separated and coded into a false color. The atomic number information is coded into the hue value of a color image in HIS (Hue, Intensity, Saturation) format. The mixed information about the thickness and the density is coded into intensity of a color. A certain percentage of X-ray energy is absorbed by the material due to a process known as electron ionization. The amount of energy absorbed depends on the density and atomic number of the material. As a result, the detected X-ray attenuation provides a picture of the absorbed energy on the irradiated objects. Due to the absorbed energy being relative to the atomic number, it can be used in the material discrimination process.
In general, the lower the atomic number, the more transparent the material is to the X-rays. Materials composed of elements with a high atomic numbers absorb radiation more effectively causing darker shadows in an X-ray image. Substances with low atomic numbers absorb less X-ray radiation, hence their shadowgraph appears a lighter color. The absorption of the X-ray radiation by a material is proportional to the degree of X-ray attenuation and is dependent on the energy of the X-ray radiation and the following material parameters: thickness, density, and atomic number
The relationship between these values can be described by:
      I    x    =            I      0        ⁢          exp      ⁡              [                              -                          (                              μ                ρ                            )                                ⁢          x                ]            Where,    Ix Intensity of the X-ray radiation after passing through a material;    I0 Intensity of the narrow beam monoenergetic X-ray radiation before passing through a material;    μ linear attenuation coefficient;    ρ material density;    x mass thickness (obtained by multiplying the thickness t by the density ρ, i.e., x=t ρ).An important component in the equation is the mass attenuation coefficient (μ/ρ), which can be rewritten:
      μ    ρ    =            x              -        1              ⁢          ln      ⁡              (                              I            0                                I            x                          )            
The mass attenuation coefficient represents the penetration and the energy deposition by the photons in materials. This can be obtained by the measurement of I0 and combination with the confirmed values of Ix and x. Research has been directed to obtaining the mass attenuation coefficient for radiological interest, as this value is characteristic for each element, mixture and compound. The dependence of the X-ray attenuation on the atomic number relies on three phenomena: photoelectric effect, Compton effect and pair production. All three mechanisms demonstrate the quantum nature of X-ray radiation.
The color in an X-ray image indicates the type of material. To produce color X-ray images, the current system employs the two energy levels. The radiation of X-ray interacts with the object under inspection causing X-ray attenuation. The attenuation of low and high X-ray energy is determined on the representative X-ray detectors and processed to produce a color image. The two different X-ray energy levels are passed through the objects, which show characteristic drops in intensity corresponding to the absorption at particular energy levels. The intensity of the generated shadow of an object at two different energy levels is unequal; it is dependent on the density as well as the material type. The combination of the measurements at the two different energy levels together with the knowledge of X-ray interaction allows for the determination of the material.
One form of inspection that has been investigated is the use of dual energy x-ray absorption scanners. Dual energy refers to radiation at two or more bands of energy, emitted simultaneously or in succession, or as part of a broadband of polyenergetic radiation over the diagnostic imaging range. As is known in the art, the measurement of x-ray energy attenuated by an object in two distinct energy bands can be used to determine information about the photoelectric absorption and Compton scattering of the particular materials of the object.
Photoelectric absorption and Compton scattering are determined by the electron density and atomic number of the materials and are functions of the x-ray energy. The photoelectric effect is predominant at low X-ray energies and with high atomic numbers. When a quantum of radiation strikes an atom, it may impinge on an electron within an inner shell and eject it from the atom. If the photon carries more energy than is necessary to eject the electron, it will transfer this residual energy to the ejected electron in the form of kinetic energy. The probability of the photoelectric effect per atom can be described by the following relationship:
  σ  ∝            Z      n              E              7        2            Where,    7=cross-section of the photon effect;    Z=atomic number of the irradiated substance;    n=varying exponent between 4 and 5 across E;    E=quantum of the X-ray energy (photon energy).
The Compton effect occurs primarily in the absorption of high X-ray energy and low atomic numbers. The effect takes place when high X-ray energy photons collide with an electron. Both particles may be deflected at an angle to the direction of the path of the incident X-ray. The incident photon having delivered some of its energy to the electron emerges with a longer wavelength. These deflections, accompanied by a charge of wavelength are known as Compton scattering. The probability of the Compton effect per atom is illustrated in Figure above and described by:
  σ  ∝      Z    E  Accordingly, with two measurements of the object and two different energies, a proportion of two predefined materials of a composition can be identified.
It is important to note that a by-product of this calculation is that the total quantity of material measured is factored out and hence this measurement process is particularly suited for industrial applications where the measured produce varies in thickness, density or is highly inhomogeneous. It is important, too, to note that the existence of only two attenuation mechanisms of Compton scattering and photoelectric absorption means that additional measurements at third or fourth x-ray energies provide no new information in this method. Techniques using more than two energy measurements, insofar as they are different from the present modeling approach, may not produce this same benefit of eliminating sample mass effects. Dual energy x-ray absorption scanners produce output intensities at two different x-ray energies in different ways. An x-ray tube working at one voltage, for example 150 keV, will produce x-rays with energies from 150 keV down to 0 keV. To select two groups of x-ray energies from this distribution, two detectors may be used where each detector is capable of measuring one of the two groups of x-ray energies required. In a dual-energy X-ray system, the high and low energy level are employed to identify materials. Metals and other heavier elements strongly absorb the low X-ray energy radiation and lighter materials including organic materials tend to strongly absorb high X-ray energy radiation. Using this method the material can be distinguished into different categories, according the atomic number.
Thus, there is a need in the art for a system providing a more efficient and manageable method for producing food products with desired properties.